Binderless storage phosphor screen comprising a support including an amorphous (a-C) carbon layer

ABSTRACT

A binderless storage phosphor screen comprises a vacuum deposited phosphor layer on a support, wherein the support includes a layer of amorphous carbon and, optionally, one or more auxilliary layers.

The application claims the benefit of U.S. provisional application No.60/394,581 filed Jul. 9, 2002

FIELD OF THE INVENTION

This invention relates to a binderless phosphor screen with a supportincluding an amorphous carbon (a-C) layer.

BACKGROUND OF THE INVENTION

A well-known use of phosphors is in the production of X-ray images. In aconventional radiographic system an X-ray radiograph is obtained byX-rays transmitted image-wise through an object and converted into lightof corresponding intensity in a so-called intensifying screen (X-rayconversion screen) wherein phosphor particles absorb the transmittedX-rays and convert them into visible light and/or ultraviolet radiationto which a photographic film is more sensitive than to the direct impactof X-rays.

According to another method of recording and reproducing an X-raypattern disclosed e.g., in U.S. Pat. No. 3,859,527 a special type ofphosphor is used, known as a photostimulable phosphor, which beingincorporated in a panel or screen, is exposed to incident pattern-wisemodulated X-ray beam and, as a result thereof, temporarily stores energycontained in the X-ray radiation pattern. At some interval after theexposure, a beam of visible or infra-red light scans the panel or screento stimulate the release of stored energy as light that is detected andconverted to sequential electrical signals which can be processed toproduce a visible image. For this purpose, the phosphor should store asmuch as possible of the incident X-ray energy and emit as little aspossible of the stored energy until stimulated by the scanning beam.This is called “digital radiography” or “Computed Radiography” (CR).

In both kinds of radiography the amount of exposure given for anexamination is often tuned by a “phototimer”. A “phototimer” comprises aradiometer for measuring the radiation dose passing through the object(patient) and the radiographic imaging system and a connection to thesource of penetrating radiation for switching the penetrating radiationsource off as soon as a pre-set dose is reached. In systems using such aphototimer it is important that a well measurable dose reaches theradiometer in the phototimer, since when the dose reaching thephototimer is too low, the reproducibility of the off-switching of thesource of penetrating radiation is not what it should be from the pointof view of image quality. Thus, the imaging system should itself onlyabsorb penetrating radiation up to such an extent as is necessary forgood speed and image quality so that—with a patient dose as low aspossible and only dictated by the examination at hand—the radiometer isreached by a sufficiently high exposure dose for reproducibleoff-switching of the source of penetrating radiation.

In a practical setting the amount of radiation that reaches the“phototimer” is determined by the absorption of penetrating radiation bythe object, the tube side of the cassette containing the storagephosphor panel or screen and the back side of the cassette. Theabsorption of the storage phosphor panel or screen is determined by thephosphor that is used, the amount of phosphor and the support. Higherabsorption in the phosphor layer is advantageous for speed and imagequality of the radiographic imaging system so there is a need toincrease the thickness (the absorption) of the phosphor layer, this canonly be done when the total absorption of phosphor layer and supportremains almost constant. Thus increasing the thickness of the phosphorlayer must be compensated by lowering the absorption of penetratingradiation in the support. Especially in radiographic techniques wherepenetrating radiation of low energy is used (e.g. mammography, certainnon-destructive testing applications, etc.) the contribution of thesupport to the absorption of the phosphor screen or panel or screen cannot be neglected.

The lowering of the absorption of penetrating radiation by the supportcan be done by lowering the thickness of the support, by using a supportwith low absorption, etc. On the other hand the support of the storagephosphor panel or screen should have high mechanical strength, lowbrittleness and, in case of vacuum deposition of the phosphor on it, beable to withstand the temperatures encountered during vapor deposition.Thus the need for a support giving a good compromise between oftencontradictory properties, as those cited above, remains present.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a storage phosphor panel orscreen including a support with low absorption of penetrating radiationthat has high mechanical strength and that can be used when applyingvapor deposition of a phosphor.

It is a further object of the invention to provide a storage phosphorpanel or screen including a support with low absorption of X-rayradiation with an energy lower than 70 keV that has high mechanicalstrength and that can be used when applying vapor deposition of aphosphor, the panel or screen being well suited for use in mammography.

The object of the invention is realized by providing a storage phosphorpanel or screen as claimed in claim 1. Specific features for preferredembodiments of the invention are disclosed in the dependent claims.

Further advantages and embodiments of the present invention will becomeapparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an embodiment of a storage phosphor panel orscreen of this invention.

FIG. 2 shows schematically a further embodiment of a storage phosphorpanel or screen of this invention.

FIG. 3 shows schematically an other embodiment of a storage phosphorpanel or screen of this invention.

DETAILED DESCRIPTION OF THE INVENTION

In the production of binderless phosphor screens by means of chemicalvapor deposition in vacuum, the support on which the phosphor isdeposited can be heated up to a temperature of about 400° C. So use of athermostable support is necessary. Therefore, though being a supportcontaining only elements with low atomic number, a polymeric support isnot the most suitable. It was now found that including an amorphouscarbon film in the support did open perspectives in order to produce abinderless storage phosphor screen on a support with low X-rayabsorption, even if the storage phosphor layer is applied by vacuumdeposition at fairly high temperatures. Amorphous carbon films suitablefor use in this invention are commercially available through, e.g.,Tokay Carbon Co, LTD of Tokyo, Japan or Nisshinbo Industries, Inc ofTokyo, Japan, where they are termed “Glass-Like Carbon Film”, or “GlassyCarbon”.

In a binderless phosphor panel or screen according to the presentinvention, the thickness of the amorphous carbon layer can range from100 μm up to 3000 μm, a thickness between 500 μm and 2000 μm beingpreferred as compromise between flexibility, strength and X-rayabsorption.

A first embodiment of the invention

In a binderless storage phosphor screen of the present invention thestorage phosphor layer can be directly positioned adjacent to theamorphous carbon layer, e.g., by vacuum depositing the storage phosphoron the amorphous carbon film, and the screen can be used without addingfurther layers to the screen, this is a very simple embodiment of astorage phosphor screen of the present invention. This embodiment isshown in FIG. 1 wherein a storage phosphor layer (1) on a support (2) isadjacent to an amorphous carbon layer (23).

A second embodiment of the invention

In a further embodiment of the storage phosphor screen or panelaccording to the present invention an auxiliary layer can be added tothe screen at the side of the amorphous carbon layer facing away fromthe phosphor layer. Such a screen is shown in FIG. 2, wherein a phosphorlayer (1) on a support (2) is schematically shown wherein the supportincludes an amorphous carbon layer (23) and an auxiliary layer (24).This auxiliary layer is preferably a polymeric layer that is laminatedto the amorphous carbon layer. By doing so the mechanical strength,especially with respect to brittleness and flexibility, of the panel orscreen of the present invention is enhanced. The need for very highmechanical strength is especially present in the radiographic systemsmaking use of a storage phosphor panel wherein during reading of theenergy stored in the panel, the panel is automatically removed from thecassette, moved through a reader, often via a sinuous path, and then putback in the cassette. In such a reader it is quite advantageous to makeuse of a screen or panel of the present invention with an auxiliarylayer laminated on the amorphous carbon layer. This auxiliary layer canbe any polymeric film known in the art, e.g. polyester film,polyvinylchloride, polycarbonate, syntactic polystyrene, etc. Preferredpolymeric films are polyester ester films, as e.g., polyethyleneterephthalate films, polyethylene naphthalate films, etc. The thicknessof the auxiliary layer (24) can range from 1 μm to 500 μm. It ispossible to use a fairly thin amorphous carbon film, e.g., 400 μm andlaminate a 500 μm thick auxiliary film to it as well as to use a thickamorphous carbon film, e.g., 2000 μm thick with a thin, e.g., 6 μmthick, polymeric film laminated onto it. The relative thickness of theamorphous carbon and polymeric film can be varied widely and is onlydirected by the required physical strength of the amorphous carbonduring deposition of the phosphor layer and the required flexibilityduring use of the panel.

A third embodiment of the invention

It has been shown, e.g. in the European Patent Application No.02100763.8 concurrently filed herewith, Jun. 28, 2002, that adding aspecularly reflecting layer between the phosphor layer and the amorphouscarbon layer can enhance both image quality and speed of the screen orpanel. Also in a panel according to the present invention, the additionof such a specularly reflecting auxiliary layer may be beneficial. Whensuch a layer is added, it preferably reflects at least 80% of the lightimpinging on it in a specular way. More preferably said layer reflects90% of the impinging light specularly. Such layers are preferably verythin (thickness under 20 μm, preferably under 10 μm) metal layers. Whenin a screen or panel according to the present invention, a specularlyreflecting layer is present, it is preferred that the layer is a thinaluminum layer (thickness preferably lower than or equal to 10 μm, morepreferably lower than or equal to 5 μm). Since such a thin metal layercan be quite corrosion sensitive it is preferred that, when a specularlyreflecting metal layer is present in a panel or screen of the presentinvention, that this layer is covered with a barrier layer (a furtherauxiliary layer) that impedes water and/or moisture of reaching therelecting auxiliary layer. Such a barrier layer can be any moisturebarrier layer known in the art, but is preferably a layer of parylene.Most preferred polymers for use in the barrier layer of the presentinvention are vacuum deposited, preferably chemical vacuum depositedpoly-p-xylylene film. A poly-p-xylylene has repeating units in the rangefrom 10 to 10000, wherein each repeating unit has an aromatic nucleargroup, whether or not substituted. As a basic agent the commerciallyavailable di-p-xylylene composition sold by the Union Carbide Co. underthe trademark “PARYLENE” is thus preferred. The preferred compositionsfor the barrier layer are the unsubstituted “PARYLENE N”, themonochlorine substituted “PARYLENE C”, the dichlorine substituted“PARYLENE D” and the “PARYLENE HT” (a completely fluorine substitutedversion of PARYLENE N, opposite to the other “parylenes” resistant toheat up to a temperature of 400° C. and also resistant to ultra-violetradiation, moisture resistance being about the same as the moistureresistance of “PARYLENE C”). Most preferred polymers for use in thepreparation of the barrier layer in a panel of this invention arepoly(p-2-chloroxylylene), i.e. PARYLENE C film,poly(p-2,6-dichloroxylylene), i.e. PARYLENE D film and “PARYLENE HT” (acompletely fluorine substituted version of PARYLENE N. The advantage ofparylene layers as moisture barrier layers in a panel or screen of thepresent invention layer is the temperature resistance of the layers, thetemperature resistance of the parylene layers is such that they canwithstand the temperature need for vacuum depositing the storagephosphor. The use of parylene layers in storage phosphor screens hasbeen disclosed in, e.g., EP-A's 1 286 362, 1 286 363, 1 286 364 and 1286 365.

Thus a screen or a panel according to this third embodiment of theinvention as set forth hereinbefore has (FIG. 3) a phosphor layer (1)and a support (2) wherein the support includes an amorphous carbon layer(23) and between the phosphor and the amorphous carbon layer aspecularly reflecting layer (22) adjacent to the amorphous carbon layerand a parylene layer (21) on top of the reflecting layer. A polymericlayer (24) is laminated to the amorphous carbon layer. In a preferredembodiment according to the present invention said reflective auxiliarylayer (22) is an aluminum layer with a thickness between 0.2 μm and 200μm.

The invention moreover includes a method for producing a storagephosphor panel comprising the steps of

providing an amorphous carbon film,

vacuum depositing a storage phosphor layer on said amorphous carbon filmand

optionally laminating a polymeric film on the side of the amorphouscarbon film not covered by said phosphor.

The invention further includes a method for producing a storage phosphorpanel comprising the steps of

providing an amorphous carbon film

applying a specularly reflecting layer on said amorphous carbon film,

vacuum depositing a storage phosphor layer on said amorphous carbon filmand

optionally laminating a polymeric film on the side of the amorphouscarbon film not covered by said phosphor.

The invention further includes a method for producing a storage phosphorpanel comprising the steps of

providing an amorphous carbon film

applying a specularly reflecting layer on said amorphous carbon film

chemical vacuum depositing a parylene layer on top of said specularlyreflecting layer,

vacuum depositing a storage phosphor layer on said amorphous carbon filmand, optionally,

laminating a polymeric film on the side of the amorphous carbon film notcovered by said phosphor.

The screen or panel of this invention can include on top of the phosphorlayer any protective layer known in the art. Especially suitable for useare those protective layers disclosed in EP-A's 1 286 363, 1 316 969 and1 316 970. Screens or panels according to the present invention, whereina moisture-repellent layer is present inbetween said substrate and saidphosphor layer are advantageously used, and, furtheron a screen or panelaccording to the present invention, wherein, adjacent to the saidphosphor layer, a moisture-repellent layer is coated as an outermostlayer is even more preferred. Especially said screens or panels havingmoisture-repellent parylene layers are recommended. Screens or panels,wherein said phosphor layer is sandwiched between two moisture-repellentparylene layers provides an excellent protection.

The screen or the panel of the present invention may also havereinforced edges as described in, e.g., U.S. Pat. No. 5,334,842 and U.S.Pat. No. 5,340,661.

The surface of the phosphor layer (1) in a panel or screen of thepresent invention can be made smaller than the surface of the support(2) so that the phosphor layer does not reach the edges of the support.Such a screen has been disclosed in, e.g., EP-A 1 286 363.

The storage phosphor used in a panel or screen of the present inventionis preferably an alkali metal storage phosphor. Such a phosphor isdisclosed in U.S. Pat. No. 5,736,069 and corresponds to the formula M¹⁺X.aM²⁺ X′₂bM³⁺ X″₃:cZ

wherein: M¹⁺ is at least one member selected from the group consistingof Li, Na, K, Cs and Rb,

M²⁺ is at least one member selected from the group consisting of Be, Mg,Ca, Sr, Ba, Zn, Cd, Cu, Pb and Ni,

M³⁺ is at least one member selected from the group consisting of Sc, Y,La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Bi, Inand Ga,

Z is at least one member selected from the group Ga¹⁺, Ge²⁺, Sn²⁺, Sb³⁺and As³⁺,

X, X′ and X″ can be the same or different and each represents a halogenatom selected from the group consisting of F, Br, Cl, 1 and 0≦a≦1, 0≦b≦1and 0<c≦0.2.

An especially preferred phosphor for use in a panel or screen of thepresent invention is a CsX:Eu stimulable phosphor, wherein X representsa halide selected from the group consisting of Br and Cl, produced by amethod comprising the steps of:

mixing said CsX with between 10⁻³ and 5 mol % of a Europium compoundselected from the group consisting of EuOX′, EuX′₂ and EuX′₃, X′ being amember selected from the group consisting of F, Cl, Br and I;

firing said mixture at a temperature above 450° C.;

cooling said mixture and

recovering the CsX:Eu phosphor.

Such a phosphor has been disclosed in EP-A-1 203 394.

The phosphor is preferably vacuum deposited on the support underconditions disclosed in EP-A-1 113 458 and EP-A-1 118 540.

In a preferred embodiment the panel or screen according to the presentinvention is a binderless phosphor panel or screen, wherein saidphosphor layer comprises a needle-shaped CsX:Eu phosphor, wherein Xrepresents a halide selected from the group consisting of Br and Cl.

The present invention moreover includes a method for exposing an objectto X-rays comprising the steps of:

providing an X-ray machine including an X-ray tube equipped for emittingX-rays with an energy lower than or equal to 70 keV and a phototimercoupled to said X-ray tube for switching said tube on and off inaccordance with an X-ray dose reaching said phototimer,

placing an object between said X-ray tube and said phototimer

placing a binderless storage phosphor panel or screen according to thisinvention between said object and said phototimer and

activating said X-ray tube for exposing said object, said cassette andsaid phototimer until said phototimer switches said X-ray tube off.

The present invention further includes a method according as describedjust hereinbefore, wherein said X-ray tube is equipped for emittingX-rays with an energy lower than or equal to 40 keV.

A screen or panel of this invention is thus very well suited for use inmammography where X-ray machines with low keV are used, and in certainnon-destructive testing applications.

PARTS LIST

-   1. phosphor layer-   2. support-   21 auxiliary layer, moisture barrier layer-   22 auxiliary layer, specularly reflecting layer-   23 amorphous carbon layer-   24 auxiliary layer, polymeric layer

EXAMPLES

While the present invention will hereinafter be described in connectionwith preferred embodiments thereof, it will be understood that it is notintended to limit the invention to those embodiments.

An X-ray cassette with a phosphor screen or panel was exposed withX-rays having an energy of 28 keV from a Mo-anode (30 μm Mo, internfiltering and without filtering, respectively).

A Mammory Detail R® screen, trade marketed product from Agfa-Gevaert,Morstel, Belgium, was taken as a comparative screen: as that system justadmits use of a “phototimer” (with respect to absorption of X-rayexposure energy as explained in the detailed description hereinbefore).Absorption for all examined cassettes with screens or panels should thusnot exceed the absorption, measured for the comparative screen, setforth hereinbefore.

Starting from a 10 mR X-ray dose reaching the cassette, X-rays passingthrough 4 cm of a polymethyl methacrylate polymeric layer, furtherconsecutively passing the cassette bottom (3 mm of polyethylene), thepanel or screen (varying composition in the experiments as will beexplained hereinafter) and the cassette cover (4.1 mm of polyethylene),it has been measured that a dose in the range from 0.75 up to 0.85 mR isrequired in order to get an acceptable and precise working of the“phototimer”, in order to avoid too much exposure to X-rays for thepatient.

In the panels or screens, CsBr:Eu phosphor layers (of varyingthicknesses, expressed in μm and indicated in the Table 1) were coatedon varying supports (aluminum, a-C “amorphous carbon”, glass and iron),having varying thicknesses (expressed in μm in the Table 1) and X-rayenergies (doses in mR) reaching the “phototimer” have been summarized inthe Table 1 for each examined panel or screen. As a thickness of thesupport layer, the thickness still offering enough dose at the positionof the phototimer after the X-rays have passed the cassette, fordiffering thicknesses of the CsBr:Eu phosphor layer, have been given inthe Table 1 hereinafter. TABLE 1 Support material and CsBr:Eu phosphorlayer Dose detected at the its thickness (μm) thickness (μm) phototimer(mR) Al 100 μm 150 μm 0.75 Al 400 μm 125 μm 0.78 Al 800 μm 100 μm 0.76a-C 2000 μm 150 μm 0.73 a-C 2000 μm 125 μm 0.81 a-C 2000 μm 100 μm 0.91Glass 2000 μm 140 μm 0.95 Glass 2000 μm 150 μm 0.85 Glass 2000 μm 160 μm0.76 Fe 100 μm  60 μm 0.55 Fe 100 μm  80 μm 0.44 Fe 100 μm 100 μm 0.36

From the results obtained in the Table 1, it is clear that the amorphouscarbon (a-C) support is superior as little absorption occurs, ifcompared e.g. with Fe (not suitable for use, even not for a layerthickness of only 100 μm) and with aluminum (suitable for use up to 800μm for a thinner phosphor layer of 100 μm): amorphous carbon providesenough dose at the position of the phototimer, even for the thickestphosphor layer (150 μm) and a thickness of 2000 μm is perfectly suitablefor use! Amorphous carbon is comparable with glass as illustrated inTable 1, but it is superior with respect to glass as it is much moresuitable to be applied in the manufacturing of phosphor panels orscreens of the present invention.

Having described in detail preferred embodiments of the currentinvention, it will now be apparent to those skilled in the art thatnumerous modifications can be made therein without departing from thescope of the invention as defined in the appending claims.

1-51. (canceled)
 52. A method for exposing an object to X-rayscomprising the steps of: providing an X-ray machine including an X-raytube equipped for emitting X-rays with an energy lower than or equal to70 keV and a phototimer coupled to said X-ray tube for switching saidtube on and off in accordance with an X-ray dose in the range from 0.75up to 0.85 mR reaching said phototimer, placing an object between saidX-ray tube and said phototimer, placing a cassette with a binderlessstorage phosphor panel or screen between said object and said phototimerand activating said X-ray tube for exposing said object, said cassetteand said phototimer until said phototimer switches said X-ray tube off,wherein said binderless storage phosphor panel comprises a vacuumdeposited phosphor layer (1) on a support (2), and wherein said supportincludes a layer of amorphous carbon (23) having a thickness between 500μm and 2000 μm.
 53. Method according to claim 52, wherein said supportfurther includes a reflective auxiliary aluminum layer (22) with athickness between 0.2 μm and 200 μm.
 54. Method according to claim 52,wherein said support further includes a protective auxiliary layer (21)between said reflective auxiliary layer and said phosphor layer. 55.Method according to claim 53, wherein said support further includes aprotective auxiliary layer (21) between said reflective auxiliary layerand said phosphor layer.
 56. Method according to claim 54, wherein saidprotective auxiliary layer (21) is a layer of parylene wherein saidparylene is selected from the group consisting of parylene C, parylene Dand parylene HT.
 57. Method according to claim 55, wherein saidprotective auxiliary layer (21) is a layer of parylene wherein saidparylene is selected from the group consisting of parylene C, parylene Dand parylene HT.
 58. Method according to claim 52, wherein said phosphorlayer comprises a needle shaped CsX:Eu phosphor, wherein X represents ahalide selected from the group consisting of Br and Cl.
 59. Methodaccording to claim 53, wherein said phosphor layer comprises a needleshaped CsX:Eu phosphor, wherein X represents a halide selected from thegroup consisting of Br and Cl.
 60. Method according to claim 54, whereinsaid phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein Xrepresents a halide selected from the group consisting of Br and Cl. 61.Method according to claim 55, wherein said phosphor layer comprises aneedle shaped CsX:Eu phosphor, wherein X represents a halide selectedfrom the group consisting of Br and Cl.
 62. Method according to claim56, wherein said phosphor layer comprises a needle shaped CsX:Euphosphor, wherein X represents a halide selected from the groupconsisting of Br and Cl.
 63. Method according to claim 57, wherein saidphosphor layer comprises a needle shaped CsX:Eu phosphor, wherein Xrepresents a halide selected from the group consisting of Br and Cl. 64.Method according to claim 52, wherein said support further includes apolymeric auxiliary layer (24) farther away from said phosphor layerthan said layer of amorphous carbon.
 65. Method according to claim 53,wherein said support further includes a polymeric auxiliary layer (24)farther away from said phosphor layer than said layer of amorphouscarbon.
 66. Method according to claim 54, wherein said support furtherincludes a polymeric auxiliary layer (24) farther away from saidphosphor layer than said layer of amorphous carbon.
 67. Method accordingto claim 55, wherein said support further includes a polymeric auxiliarylayer (24) farther away from said phosphor layer than said layer ofamorphous carbon.
 68. Method according to claim 56, wherein said supportfurther includes a polymeric auxiliary layer (24) farther away from saidphosphor layer than said layer of amorphous carbon.
 69. Method accordingto claim 57, wherein said support further includes a polymeric auxiliarylayer (24) farther away from said phosphor layer than said layer ofamorphous carbon.
 70. Method according to claim 58, wherein said supportfurther includes a polymeric auxiliary layer (24) farther away from saidphosphor layer than said layer of amorphous carbon.
 71. Method accordingto claim 59, wherein said support further includes a polymeric auxiliarylayer (24) farther away from said phosphor layer than said layer ofamorphous carbon.
 72. Method according to claim 60, wherein said supportfurther includes a polymeric auxiliary layer (24) farther away from saidphosphor layer than said layer of amorphous carbon.
 73. Method accordingto claim 61, wherein said support further includes a polymeric auxiliarylayer (24) farther away from said phosphor layer than said layer ofamorphous carbon.
 74. Method according to claim 62, wherein said supportfurther includes a polymeric auxiliary layer (24) farther away from saidphosphor layer than said layer of amorphous carbon.
 75. Method accordingto claim 63, wherein said support further includes a polymeric auxiliarylayer (24) farther away from said phosphor layer than said layer ofamorphous carbon.
 76. Method according to claim 52, wherein said methodis a mammographic application method.